Low-density bonded nonwoven fabrics and process therefor

ABSTRACT

FIBROUS WEBS IN WHICH THE FIBERS ARE FREE TO MOVE RELATIVE TO EACH OTHER ARE BONDED WITH FLUID POLYMERIC BINDER SOLUTIONS OR DISPERSIONS CONTAINING A PROPORTION OF MICROSPHERIC SPHERES OF EXPANDABLE PLASTIC FILLED WITH AN EXPANDABLE LOW-BOILING LIQUID OR VAPOR. IN DRYING THE FLUID FROM THE BINDER, THE MICROSPHERES ARE EXPANDED IN VOLUME BUT NOT RUPTURED, LEADING TO THE FORMATION OF NONWOVEN FABRICS WITH LOW DENSITY AND HIGH FLUID ABSORBENCY.

July 11, 1972 A. e. HOYLE 3,676,288

LOW-DENSITY BONDEJ NONWOVEN FABRICS AND PROCESS THEREFOR Filed May 4,1970 United States Patent 3,676,288 LOW-DENSITY BONDED NONWOVEN FABRICSAND PROCESS THEREFOR Albert G. Hoyle, Lowell, Mass., assignor to TheKendall Company, Boston, Mass. Filed May 4, 1970, Ser. No. 34,493 Int.Cl. B32b 5/02, 5/22 US. Cl. 161-158 7 Claims ABSTRACT OF THE DISCLOSUREThis invention relates to a method for improving the insulating value,opacity, thickness, and absorbency of nonwoven fabrics. Morespecifically, it relates to a method of producing bonded nonwovenfabrics of exceptionally low density and high absorptive power, suitedfor use as surgical dressings, disposable dusters, wiping cloths,blankets, interlinings, and the like.

Nonwoven fabrics of various types are finding rapidly increasing usageas surgical dressings, dusting, cleaning, and polishing cloths, as fluidor paste applicators, and for a variety of allied uses. In suchapplications they are generally classified as disposable or limited useitems, and are preferred over Woven fabrics for reasons of convenienceand economy. The majority of such fabrics are composed of rayon orcotton fibers, or mixtures thereof, carded or garnetted into a fleeceand then bonded with an overall or a patterned application of ,apolymeric binder.

Such prior art products, however, are not ideal in all respects. Intheir behavior when folded or crumpled in use they are apt to bepaper-like, lacking in the easy conformability of woven or knittedfabrics. Furthermore, the inter-fiber spacing is generally of smalldimensions, the fibers being closer together and in a more compactedconfiguration than is desirable. Close fiber-to-fiber contact promoteshigh capillarity in an absorbent fibrous article, but decreases thepotential total absorbency or pick-up. When it is desired that anonwoven surgical dressing pick-up and retain large amounts of fluid, afabric of low density, with substantial spaces or pores between fibers,is preferred. Similar considerations pertain when nonwoven fabrics areintended for use as dusters or wiping cloths, both for efficiency inpick-up and in case of shaking or washing the fabric clean after use.

Also, the insulating value of conventional nonwoven fabrics leavessomething to be desired, since the close inter-fiber spacing minimizesthe dead air space essential to proper insulation required forlimited-use blankets, sleeping bag liners, and the like.

One possible reason for the undesirably high density of prior-art bondednonwoven fabrics is that polymeric bonding agents are usually applied tofibrous webs in the form of aqueous emulsions or dispersions containing15% to 25% solids. Although delivered as 40-50% solids content, thedispersions are too viscous in that form for application to delicatefibrous fleeces, and must be diluted.

The fluid binding material, held in between the fibers in the form ofdroplets, loses 75% to 85% of its volume during drying, with aconsequent drawing together and compaction of the fibers and a decreasein the inter-fiber lot:

spacing. Loft and absorbency are sacrified, and as an additionaldisadvantage, most bonded nonwoven fabrics have a translucency orsemi-transparency that is undesirable in blankets, sheets, draperies,and the like.

It is with improvements in producing non-compacting polymeric bindersystems that the present invention is concerned.

It is a primary object of this invention to provide a fluid-dispersedpolymeric binder system for nonwoven fabrics in which the binder expandsduring the drying operation.

It is a further object of this invention to provide nonwoven fabrics ofexceptionally low density and high opacity, suitable for use as surgicaldressings, disposable dusters, all-purpose cleaning cloths, blankets,and the like.

The basis of this inveniton lies in the incorporation into afluid-dispersed polymeric binder, such as a dilute aqueous latex, of aproportion of microspheres of expandable plasitc, said spheres enclosinga heat-expandable fluid which increases drastically in volume attemperatures at which nonwoven fabrics are dried; impregnating a loose,unbonded fibrous fleece with such a mixture, said fleece comprisingintermingled and overlapping fibers in casual mechanical engagement; anddrying the impregnated fleece at a temperature suflicient to expand butnot to rupture the plastic spheres, thus forming a closed-cell expandedbinder system uniting the fibers.

One such type of microspheres, available commercially, is composed ofminute spheres of a copolymer of vinylidene chloride and acrylonitrileor of an acrylic polymer enclosing a nucleus of isobutane, which is agas at room temperture, with a boiling point of l0 C. The microspheresat room temperature are in the form of a fine dry powder, the individualparticles of which average 6 to 8 microns in diameter. When heated to F.or higher, however, the tendency of the isobutane nucleus to expand,coupled with the gradual softening of the plastic shell, forms expandedspheres with diame ters averaging 25-30 microns, said spheres containingisobutane vapor. The expanded spheres therefore have a volume which ison the average over 60 times the volume which they occupied inunexpanded form. The particular temperature at which expansion occurswill vary with the nature of the plastic shell, a convenient workingrange being 190 F.250 F.

In the practice of this invention, two important criteria should beobserved. First, the fibers in the fleece should be in only casualmechanical engagement, with maximum fiber mobility in all directions tomove in compliance with the expanding binder-microsphere combination.Second, the drying or curing temperature of the saturated fleece shouldbe such that the microspheres are expanded, but remain intact spheresand are not ruptured. By following these criteria, closed-cellstructures are obtained, with three-fold to six-fold increases inthickness.

The invention will be more fully understood from the followingdescription and drawings, in which:

FIG. 1 is an idealized view of segments of two interlaced fibers bondedby a prio-artpolymeric binder.

FIG. 2 is a view of segments of two similar fibers bonded by the bindersystem of this invention.

Referring to FIG. 1, a pair of intercrossing fibers 10 and 12 are shownas bonded into a part of a nonwoven fabric by a bead 14 of driedpolymeric bonding material. It should be appreciated that FIG. 1 ishighly magnified, and simplified in the sense that a nonwoven fabricwill show the fibers interlaced and entangled in a more complex fashion,although each fiber is still displaced with relative ease from itsengagement with other fibers. It is nevertheless representative in thatwhen a bead or droplet of aqueous polymeric binder is deposited betweentwo or more fibers, it comes to rest in a position where its tendency tospread by capillary action is equilibrated with surface tension forcesresisting spread. Frequently this results in a lenticular bead of binderwetting a pair of fibers, as shown. As the web is dried, the volume ofthe bead of binder shrinks by loss of water, as explained above. The netresult is a drawing or holding together of the fibers in a tightlycompacted configuration, with a consequent loss of absorbency and anincrease in density.

FIG. 2 is a similarly magnified view of a pair of fibers 16 and 18 heldtogether by a bead of binder 20 which contains numerous expanded butintact gas-filled micro spheres 22. Instead of the fibers 16 and 18having been drawn together compactly in the drying process, theexpanding tendency of the microspheres during the drying of the nonwovenfabric has increased the volume of the head of binder. Such an expandedbead, although it still serves to bind the fibers together, is much morevoluminous and of lower density than a normal head of binder notcontaining microspheres.

As starting material in the practice of this invention there is employeda fleece of intermingled and overlapping fibers, preferably comprising asubstantial proportion of fibers of textile length. By textile length ismeant those fibers, usually averaging one-half inch in length or longer,which can be dry-assembled into a fleece or web by conventional textilefiber-separation devices such as cards, garnetts, air-lay machines, andthe like. For economy, and particularly where high water absorbency isdesired, rayon or cotton fibers are the fibers of choice, although allof the synthetic fibers of commerce may be employed, such as nylon,acrylic, polyester, and polyolefin fibers. The textilelength fibers mayfor economy be blended with shorter fibers such as papermaking fibers orflock, such blends being amenable to dry-deposition processes.

Similarly, the particular polymeric bonding agent employed is notcritical, being varied in accordance with the particular propertiesdesired in the final product. Polymeric dispersions of acrylates ormethacrylates are very often the choice where soft and absorbentnonwoven fabrics are to be produced: however, the invention isapplicable to a variety of other polymeric binder dispersions such aspolyvinyl acetate, copolymers of styrene-butadienesacrylonitrile,natural rubber latices, and in general any polymeric dispersions whichwill wet out and adhere to the particular fibrous substrate beingemployed.

In practicing this invention, the desired proportion of microspheres inunexpanded form is mechanically stirred into the selected and dilutedaqueous polymeric binder. In testing a number of different polymericbinder dispersions, no incompatibility or adverse affect has been noted,the microspheres being inert. The binder-microsphere dispersion is thenapplied by conventional means to a fleece of intermingled overlappingfibers, as by overall saturation, line bonding, spot bonding, or anyknown technique common in the art of producing bonded nonwoven fabrics.Drying of the wet fleece is accomplished by the application of heat,steam-heated dry cans being a convenient device. As might be expected,simple air-drying does not expand the microspheres, and the desiredincreases in thickness and absorbency of the nonwoven: product are notrealized.

Appreciable increases in thickness and absorbency of nonwoven fabricsare realized when microspheres are added to a polymeric binder in aproportion of 2 parts binder to one part microspheres. As the ratio isdecreased to 1 to 1 or 1 to 2, greater expansion of the beads of binderis noted, and the thickness of the product is increased. Below a ratioof one part binder to two parts microspheres, the tensile strength ofthe end product decreases, although for special purposes a formulationof one part binder to four or even eight parts microspheres may beemployed. A workable range is one part of polymeric bonding agent tofrom one-half to four parts of .4 microspheres, with a preferred rangeof one part polymeric bonding agent to one to two parts of microspheres.

Due to the low density of the microspheres (less than unity), they maytend to float or cream in dilute dispersions of latex binders. It hasbeen found advisable as a general practice, therefore, to add to thebinder-microsphere dispersion a small amount of a thickening orsuspending agent. In the examples set forth below, for an aqueous bindersolution containing 20% total solids (polymer plus microspheres) therewas added in each instance 0.6%, based on solution weight, of a highviscosity, coldwater-soluble methyl cellulose.

The invention will be illustrated by the following examples.

EXAMPLE 1 To illustrate the need for an elevated drying tempera ture(generally 190 F. or over) to expand the microspheres, a carded fleeceof 3 denier, 1%; inch viscose rayon fibers weighing 26 grams per squareyard was saturated with an aqueous binder formulation of 20% solidsconsisting of equal parts of an acrylic latex and polyvinylidenechloride-acrylonitrile-isobutane microspheres, from Dow ChemicalCorporation, with a trace of methyl cellulose for thickening. The drypickup was 35%, and the final product weighed 35 grams per square yard.

One portion of the above product was allowed to dry at room temperature.It was 18 mils thick when dry, thickness being measured by an Ames gaugewith a pressor foot 1.5 inches in diameter. A similar portion was driedin an oven heated to 240 F. It was 60 mils thick, or 333% of thethickness of the air-dried sample. This increase in thickness wasreflected in the relative absorbencies of the two products, being 566%in the case of the 18 mil air-dried product and 1140% in the case of the60 mil steam-dried product. Absorbency was measured by immersing aweighed sample in water for five minutes, draining the sample for twominutes, weighing, and calculating the water-pickup (Absorbency test4.41, U.S. Army Natick Laboratories, Limited Purchase Description,Chamois, Synthetic).

EXAMPLE 2 This example consists of a set of four samples, all based on astarting fleece of carded 3 denier, 1% inch viscose rayon fibers. Allsamples were bonded in an overall saturation and squeeze process.

Web Micro- Wet weight, gm. Binder, spheres, pickup, per sq. yd. percentpercent percent Sample 25 10 253 B 29 1O 10 O 23. 6 5 10 246 D 22. 6 2.510 208 Weight, Percent gm. per Thickness, M.D C.D. water sq. yd. mils.tensile tensile pickup NOTE.M.D. and CD. stand for machine direction andcross direction respectively, thefigures in thetable being poundstensile strength per inchwide strip, in the dry state. The percent waterpickup is grams of water absorbed per gram of fabric, tested as inExample 1.

An additional and unexpected advantage of the process of the process ofthis invention is that when the fabrics are tested wet, there is ingeneral an 80% retention of tensile strength, versus a 40% retention oftensile strength shown by similar bonded fabrics not containingmicrospheres.

The actual percentage of binder, based on the weight of startingrayonfiber, was A, 25%; B, 15%; C, 12%; D,

Attempts to use microspheres alone, in aqueous suspension, as a binderfor carded rayon fleeces resulted in products with no measurable tensilestrength at the temperatures employed in this invention. Sincemlcrospheres alone do not bond fibers, the conclusion is that theretention of strength shown in the above table is due to a moreefiicient utilization of the actual binder material employed. As anillustration, a comparison of Samples A and C 1n the above tableindicates that although C contains less than half as much actual binderas A, the tensile strength of C is equal to or superior to that of A.That is, by the process of this invention, a given weight of polymericbinder is increased in volume so that it effectively ties together morefibers, or longer lengths of fibers, or both. In general, as a nonwovenfabric binder is diluted with nonbonding microspheres up to a ratio ofone part binder: four parts microspheres, the thickness of the resultingfabric is increased up to six-fold, with a three to five-fold increasein the water absorptive power of the fabric.

As mentioned above, the employment of microspheres in conjunction with apolymeric dispersion of binding agent for nonwoven fabrics leads to avery desirable increase in the opacity or covering power of the product.Opacity may be related to values measured by the Bausch and LombOpacimeter 33-88-12, as described in Textile Research Journal, volume38, No. 1, January 1968, page 8. Therein, a value for 64 x 60 cottonsheeting is given as 0.63. For a nylon-rayon nonwoven fabric weighing 67grams per square yard the given value is 0.44.

In a typical example, a carded fleece of viscose rayon fibers was bondedwith an acrylic latex to give a nonwoven fabric weighing 56 grams persquare yard, consisting of 80% fiber and 20% binder. The Bausch and Lombopacity was 0.61. A similar fieece was bonded with the same bondingagent-extended with microspheres to give a fabric weighing 58 grams persquare yard, consisting of 76% fiber, 8% binder, and 16% microspheres.The opacity of the fabric containing microspheres was 0.91.

It might be expected that the process of this invention, expanding thefibrous structure and increasing the absorbency as it does, would leadto nonwoven fabrics of increased air permeability. Quite unexpectedly,this does not turn out to be the case. Air porosity or air permeabilityis lower in those products made with binder-microsphere combinationsaccording to this invention than it is in those products made withbinder alone, as described in Example 3.

EXAMPLE 3 A carded fleece of 1.5 denier, 1%, inch viscose rayon fibersweighing 16 grams per square yard was saturated with a 20% aqueousdispersion of a commercial acrylic binder latex, squeezed to about 200%wet pickup, and dried on steam cans at below 250 F. The final weight was21 grams per square yard; thickness 7 mils, air porosity 1120 cubic feetof air per minute per square foot of fabric at 0.5 inch hydrostatichead.

A comparable fieece was similarly bonded using a dispersion of one partacrylic binder to two parts of microspheres. The final weight was 21grams per square yard; thickness 20 mils; air porosity 685 cubic feet ofair per minute.

Repeated testing of other similarly paired fabrics confirmed theunexpected conclusion that the increased loft and absorbency associatedwith the use of microspheres was nevertheless accompanied by a decreasein air permeability and therefore an increase in the thermal insulatingvalue (dead air space) of the fabric. Presumably this is associated withthe closed-cell nature of the expanded polymeric binder, and the factthat the non-porous binder fraction of the fabric has been expanded tooccupy a proportionately larger fraction of the fabric volume.

Polymeric binders for nonwoven fabrics have, in general, a densitygreater than 1.0 gram per cubic centimeter in the form of a dried filmafter the associated water or solvent has been evaporated. By expandingthe binder through the use of microspheres as set forth above,measurements on expanded binder films show densities of 0.6 gram percubic centimeter or lower. This leads to the production of nonwovenfabrics with an apparent density of from 0.025 to 0.075 gram per cubiccentimeter, compared with control fabrics made with unexpanded binder ofa density of 0.15 gram per cubic centimeter and higher.

As discussed above, for the preservation of maximum loft and minimumdensity of the expanded binder mass, care should be taken to avoidexposure to temperatures at which the plastic binder spheres melt. Ifexpanded spheres of polyvinylidene chloride-acrylonitrile are heated forany substantial time at 280 F., for example, the plastic spheres meltand the spongy nature of the expanded binder collapses, with aconsequent thickening of the nonwoven fabric. When Sample B of Example 2was heated to 280 F. for 2. minutes, the thickness of the fabricdecreased from 55 mils to 25 mils, with a consequent decrease inabsorbency and a substantially complete disappearance of the expandedmicrospheres.

Although the above description relates to products made by an overallsaturation with binders containing microspheres, the process of thisinvention is equally adaptable to spot-bonding, line-bonding, and theapplication of binder material to discrete and spaced-apart web areas ingeneral, with the creating of differentially-lofted and novelthree-dimensional effects. A converse effect may be ob tained by bondinga web with a conventional binder dispersion in a set of spaced-apartareas, drying the web, and then conducting a second bonding process,according to the principles set forth herein, using a microspherebinderdispersion which on heating will expand only the previously unbondedportions of the Web.

Having thus described my invention, I claim:

1. The process of producing bonded nonwoven fabrics of enhancedthickness, low density, enhanced opacity, and high water absorptivepower which comprises assembling an unspun and unwoven array ofintermingled fibers comprising a substantial proportion oftextile-length fibers, said fibers being in casual mechanical engagementonly,

applying to a portion at least of said fibers a mixture of afluid-dispersed polymeric binder containing admixed therewith aproportion of very fine plastic spheres,

said spheres comprising a thermoplastic heat-expandable generallyspherical shell enclosing a heat-expandable low-boiling organic fluid,

drying the impregnated fibrous array at a temperature sufficient tocause said plastic shells to soften and become larger in diameter due tothe expanding force of the fluid contained therein, but insufiicient tocause rupture and collapse of said shells,

and cooling said impregnated fibrous array while maint-aining saidplastic spheres in permanently expanded condition.

2. The process according to claim 1 in which between one and two partsof plastic spheres are mixed with each part of polymeric binder.

3. The process according to claim 1 where the plastic spheres are formedfrom a polyvinylidene chloride-acrylonitrile shell enclosing a portionof isobutane, and the drying temperature is between F. and 250 F.

4. The process according to claim 1 in which the polymeric binder isexpanded to a density of not greater than 0.6 gram per cubic centimeter.

5. A low-density nonwoven fabric of enhanced water absorptive powerwhich comprises an array of intermingled, unspun and unwoven fibers,

comprising a substantial proportion of textile-length fibers, bonded bya polymeric binding agent,

said polymeric bonding agent having associated and admixed therewith aproportion of expanded and unruptured plastic spheres containing anexpanded organic fluid in vapor form.

6. The product according to claim 5 in which the nonwoven fabric has anapparent density of between 0.025 and 0.075 gram per cubic centimeter.

7. The product according to claim Sin which the plastic spheres average25 to 30 microns in diameter.

References Cited UNITED STATES PATENTS Veatch 21-605 Eichel 16l162Miller 161162 Hochberg 162157 Moorman 117-26 Alford 117-28 Alford et al161DIG 5 MORRIS SUSSMAN, Primary Examiner US. Cl. X.R.

15 16ll62; 156-77; 161--159; 1l7140 A, 16

